If the PR_EMSMDB_CRED_USERNAME property is not present in user profiles, some users may experience too many calls to the user principal name (UPN) lookup. To skip the UPN lookup, install one of the following articles and then follow the steps:
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Description of the security update for Outlook 2013: June 12, 2018
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Description of the security update for Outlook 2016: June 12, 2018
Important:
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Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in
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case problems occur.
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Exit all Microsoft Office applications.
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Start Registry Editor:
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In Windows 10, go to Start, enter regedit in the Search Windows box, and then select regedit.exe in the search results.
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In Windows 8 and Windows 8.1, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
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In Windows 7, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
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Locate and then select the following registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\AutoDiscover\
Note In this registry path, <xx.0> corresponds to the Outlook version (16.0 = Outlook 2016, 15.0 = Outlook 2013).
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On the Edit menu, point to New, and then click DWORD Value.
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Type AutodiscoverDisableUPNLookup, and then press Enter.
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In the Details pane, right-click AutodiscoverDisableUPNLookup, and then click Modify.
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Select Hexadecimal for the base, type 1 in the Value data box, and then click OK.
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Exit Registry Editor.
